1. Technical Field
The present invention relates to an aromatic polyamide film laminate suitable for use as a base film for magnetic recording media, a production process therefor and high-density magnetic recording media produced therefrom. More specifically, it relates to an aromatic polyamide film laminate which is excellent in handling properties such as strength, slipperiness and winding property, abrasion resistance, performance stability under a high-temperature and high-humidity environment, and running durability; a production process therefor; and high-density magnetic recording media produced therefrom.
2. Background Art
In recent years, remarkable progress has been made in high-density magnetic recording media, as exemplified by the development and implementation of a metal thin film magnetic recording medium in which a ferromagnetic metal thin film is formed on a non-magnetic substrate by a physical deposition method such as vacuum vapor deposition or sputtering, or a plating method, and a thin layer-coated magnetic recording medium in which a needle-shaped magnetic powder such as metal powder or iron oxide powder is coated on a film to a thickness of not larger than 2 xcexcm. Known examples of the former include a Co-deposited tape (see JP-A 54-147010) and a vertical magnetic recording medium composed of Coxe2x80x94Cr alloy (see JP-A 52-134706). Known examples of the latter include an extremely thin layer-coated magnetic recording medium for high-density magnetic recording (see xe2x80x9cTechnical Report MR 94-78xe2x80x9d (1995-02) issued by the Institute of Electronics and Communication Engineering of Japan).
Since a coated magnetic recording medium of the prior art (magnetic recording medium in which a mixture of magnetic powder and an organic polymer binder is coated on a non-magnetic substrate) has a low recording density and a long recording wavelength, the thickness of its magnetic layer is as large as 2 xcexcm or more. On the other hand, a ferromagnetic metal thin film formed by thin film forming means such as vacuum vapor deposition, sputtering or ion plating has as extremely small a thickness as 0.2 xcexcm or less. In the case of an extremely thin layer-coated medium, a coated magnetic layer is as extremely thin as 0.13 xcexcm though a non-magnetic primary coat layer is provided.
In the above high-density magnetic recording media, the surface condition of the non-magnetic substrate (base film) has a great influence on the surface properties of the magnetic layer. Particularly, in the case of a metal thin film magnetic recording medium, the surface condition of the non-magnetic substrate is directly reflected on the surface of the magnetic layer (magnetic recording layer) as unevenness, which causes noise in recording and reproduction signals. Therefore, it is desirable that the surface of the non-magnetic substrate be as smooth as possible.
On the other hand, in view of the formation of the non-magnetic substrate (base film) and the handling properties such as scratching, winding and unwinding thereof in the film formation step, when the surface of the film is too smooth, slipperiness between films deteriorates, a blocking phenomenon occurs, the form of the rolled film (roll formation) is bad, with the result of a reduction in product yield and an increase in the production cost of a product. Therefore, from the viewpoint of production cost, it is desirable that the surface of the non-magnetic substrate (base film) be as rough as possible.
To attain both of the above characteristics on the surface of the base film which are antipodal to each other, it is necessary to form protrusions on the surface of the base film that have appropriate height and density derived from particles having optimized particle diameters.
As a method of forming protrusions on the surface of an aromatic polyamide film, there have been proposed (a) a method in which a predetermined amount of inorganic particles is added (JP-A 61-246919), (b) a method in which organic polymer particles or inorganic particles whose surfaces have been treated with organic polymers are added (JP-A 8-203064) have been proposed. However, it is difficult to attain flatness and handling properties at the same time by the above method of adding particles and the like to a single resin layer to form protrusions and, particularly, it is extremely difficult to avoid defects occurring at the time of winding up the film into a roll form as long as the method is used. To solve this problem, there has been proposed an aromatic polyamide film or aromatic polyimide film one surface of which has different surface roughness than the other and which is produced by laminating at least two resin layers (JP-A 1-247162). In this case, however, externally added particles, which are used to form protrusions, are liable to agglomerate in the slurry added, so that particles are fallen off from the protrusions by friction when a guide pin actually makes contact with the surface of the film, damaging the step. The film also has another defect that coarse protrusions on the surface which are derived from agglomerates of particles interfere with attaining surface flatness that provides excellent electromagnetic conversion characteristics when it is used for a high-density magnetic recording medium such as a metal thin film magnetic recording medium.
Further, an aromatic polyamide resin generates hydrogen chloride in its polymerization reaction, which must be neutralized. If the salt produced by the neutralization is not completely removed when a film is formed, for a metal thin film magnetic recording medium, the thin magnetic metal film is damaged particularly under a high-temperature and high-humidity environment, so that the film cannot maintain stable performance for a long time.
Problem to Be Solved By the Invention
It is the first object of the present invention to solve the problems of prior art and provide an aromatic polyamide film laminate suitable for use as a base film for a magnetic recording medium, which attains high abrasion resistance and high flatness at the same time while having excellent handling properties such as strength, slipperiness and winding property and which also exhibits excellent performance stability under a high-temperature and high-humidity environment, and a production process therefor.
It is the second object of the present invention to provide a metal-thin-film high-density magnetic recording medium and a multiple-coat high-density magnetic recording medium produced from the aromatic polyamide film laminate that have excellent electromagnetic characteristics and are suitable for high-density recording.
Means for Solving Problem
According to the studies of the present inventors, it has been found that the objects of the present invention can be attained by a film laminate that comprises at least two biaxially oriented films formed of an aromatic polyamide, wherein:
(1) the outermost layer of the laminate, film layer A, contains inorganic particles produced from hydroxides or carbonates of at least one metal selected from the groups Ia and IIa of the periodic table, the inorganic particles having an average particle diameter of 80 to 1,500 nm and the surface roughness (RaA) of the film layer A being 1 to 20 nm;
(2) the surface opposite to the layer A of the laminate, film layer B, has a surface roughness (RaB) of 0.1 to 10 nm; and
(3) the surface roughness (RaB) of the film layer B is smaller than the surface roughness (RaA) of the film layer A by 1 nm or more.
The aromatic polyamide film laminate of the present invention will be described in more detail hereinafter.